The present invention relates generally to the process of in-mold coating. More particularly, the present invention relates to a method and system for improving the efficiency of the in-mold coating process by predicting the fill pattern and coating pressure distribution, which information is used to provide guidance on locating nozzles and the minimum clamping pressure needed. The present invention was developed for the coating of thermoplastics. It is to be appreciated, however, that the invention may relate to other similar environments and applications, including molded articles made from thermosets.
Molded thermoplastic and thermoset articles, such as those made from polyolefins, polycarbonates, polyesters, polystyrenes and polyurethanes, are Utilized in numerous applications including those for automotive, marine, recreation, construction, office products, and outdoor equipment industries. Often, application of a surface coating to a molded thermoplastic or thermoset article is desirable. For example, molded articles may be used as one part in multi-part assemblies. To match the finish of the other parts in such assemblies, the molded articles may require application of a surface coating that has the same finish properties as the other parts. Coatings may also be used to improve surface properties of the molded article such as uniformity of appearance, gloss, scratch resistance, chemical resistance, weatherability, and the like. Also, surface coatings may be used to facilitate adhesion between the molded article and a separate finish coat to be later applied thereto.
Numerous techniques to apply surface coatings to molded articles have been developed. Many of these involve applying a surface coating to molded articles after they are removed from their molds. These techniques are often multi-step processes involving surface preparation followed by spray-coating the prepared surface with paint or other finishes. In contrast, in-mold coating (IMC) provides a means of applying a surface coating to a molded article prior to its ejection from the mold.
IMC is carried out by injecting a liquid low viscosity thermoset material onto the surface of a substrate while it is still in the mold. The coating will then solidify and adhere to the substrate. The IMC process is thus integrated with conventional thermoplastic injection molding to improve the part surface quality and to protect it from outdoor exposure.
IMC has been successfully used for many years for exterior body panels made from compression molded sheet molding compound (SMC) to improve the surface quality of SMC moldings in terms of functional and cosmetic properties. When injected onto a cured SMC part, IMC cures and bonds to provide a paint-like surface.
Historically, much work with IMCs has been done on molded articles made from thermosets. Thermosets such as phenolics, epoxies, cross-linked polyesters, and the like, are a class of plastic composite materials that are chemically reactive in their fluid state and are set or cured by a reaction that causes cross-linking of the polymer chains. Once cured, subsequent heating may soften a thermoset but will not restore it to a fluid state.
Because of its distinct advantages, IMC is now being considered for injection-molded thermoplastic parts. Thermoplastics are a class of plastic materials that can be melted, cooled to a solid form, and repeatedly re-melted and solidified. The physical and chemical properties of many thermoplastic materials, together with their ease of moldability, make them materials of choice in numerous applications in the automotive, marine, recreation, construction, office products, outdoor equipment and other fields.
Similar to IMC for SMC, IMC for thermoplastics could be used either as a topcoat or as a primer. As a topcoat, IMC is applied to the end-use exterior surface of the thermoplastic substrate, either to improve its surface appearance to automotive levels or to protect the part from outdoor exposure or both. As a primer, IMC is applied to the exterior surface of the thermoplastic part before it is painted. IMC takes the place of the adhesion promoters now applied to the plastic parts, prior to painting. IMC is much more environmentally benign than the currently used adhesion promoters.
Processes have been developed wherein a fluid coating is injected onto and dispersed over the surface of a molded part and cured. A common method of injecting a fluid IMC onto the surface of a molded article involves curing (if a thermoset material) and cooling an article in the mold to the point that it has hardened sufficiently to accept the coating, reducing the pressure against the telescoping mold half to crack open or part the mold, injecting the fluid coating, and re-pressurizing the mold to distribute the coating over the surface of the molded article. The cracking or parting of the mold involves releasing the pressure exerted on the telescoping mold half to sufficiently move it away from the molded article, thereby creating a gap between the surface of the part and the telescoping mold half. The gap allows coating to be injected onto the surface of the part without needing to remove the part from the mold.
Other processes, such as injection molding, may require that pressure be maintained on the movable mold half so as to keep the cavity closed and to prevent resin from escaping along the parting line. Further, maintaining pressure on the resin material during molding, which also requires keeping the cavity closed, often is necessary to assist in providing a more uniform crystalline or molecular structure in the molded article. Without such packing, physical properties of the molded article tend to be impaired.
In addition to the problem of resin escaping along the parting line, packing constraints can sometimes create other problems when an IMC composition is to be injected into a mold containing a molded article. Specifically, some commercially available IMCs are generally thermoset materials that cure by the application of heat. Curing of these compositions is often achieved through transfer of residual heat from the molded article. If the coating composition were to be injected after a molded article has been sufficiently cooled (packed) to allow the mold to be depressurized and parted or cracked, the molded article may lack sufficient residual heat to cure the coating. Thus, for coating compositions designed to cure on an article, it is desirably injected prior to depressurizing the mold.
Because injection molding does not permit the mold to be parted or cracked prior to injection of the IMC composition into the mold cavity, the IMC composition must be injected under sufficient pressure to compress the article in all areas to be coated. The compressibility of the molded article dictates how and where the IMC composition covers it. The process of coating an injection molded article with a liquid IMC composition is described in U.S. Pat. No. 6,617,033.
The coating process consists generally of filling, packing and curing stages. During the filling stage, once the thermoplastic substrate has been solidified, but before demolding, IMC is injected into the mold and flows forward by compressing the substrate until the exterior surface of the substrate is completely covered. More coating material is injected into the cavity during the IMC packing stage to obtain a desired coating thickness. When the coating cures completely by chemical reaction, the thermoplastic part with IMC can be ejected out of the mold.
IMC nozzle location is a concern during the mold design of IMC for thermoplastic parts. It should be cosmetically acceptable since the injection location usually leaves a mark on the surface. It should guarantee 100% part coverage and minimize the potential for trapping air. Most of the molds used in the injection molding process do not have shear edges. Therefore, if the coating injection pressure exceeds the clamping pressure during the IMC process, mold opens, leading to coating leakage. As a result, controlling the coating injection pressure to avoid leakage is a big issue.
Thus, there is a need for a computer simulation tool to predict the fill pattern and coating pressure distribution, which information will, in turn, be used to provide guidance on locating IMC nozzles and to predict the minimum clamping pressure needed to avoid coating leakage.